The method of forming images by diffusion transfer using silver salts such as silver halides is well known. In practice, this method involves, for example, processing a photosensitive silver halide emulsion which has been subjected to an imagewise exposure with an aqueous alkaline solution which contains a developing agent, a silver halide solvent and a film forming agent (viscosity increasing agent), reducing the exposed silver halide grains to silver by means of the developing agent, forming the residual unexposed silver halide into a transferable silver complex salt at the same time by means of the silver halide solvent, diffusing and transferring the silver complex salt by imbibition to a silver precipitant containing layer (image receiving layer) which is superimposed on the aforementioned emulsion layer and there reducing the silver complex salt with a developing agent with the assistance of the silver precipitant and forming a silver image.
In this method, use is made, for example, of a film unit wherein a photosensitive element in which a photosensitive silver halide emulsion layer is provided on an ordinary support and an image receiving element wherein an image receiving layer which contains a silver precipitant is provided on a support are combined with a processing element which contains an active alkaline aqueous solution which contains a developing agent, a silver halide solvent and a film forming agent.
First of all, after subjecting the emulsion layer of the photosensitive element to an imagewise exposure, the photosensitive element is superimposed on the image receiving element in such a way that the emulsion layer is facing the image receiving layer of the image receiving element, and the assembly is passed between a pair of rollers in such a way that the viscous aqueous alkaline solution of the processing element is spread between the two elements. Then, after the assembly has been left to stand for a prescribed period of time, the image receiving element is peeled away from the photosensitive element, whereupon a print, which has the prescribed image formed in the image receiving layer, is obtained.
Stabilization of photographic performance in respect of the storage of photographic products in which the silver salt diffusion transfer method is used is usually considered in terms of the image receiving element, the photosensitive element and the processing element individually, but depending on the formulation of the processing composition, photographic performance inevitably deteriorates during storage. On the other hand, although measures taken only in connection with the processing composition may provide stabilization under the highly alkaline conditions in which the chemical components are active, these measures may also have an effect not only on the photosensitive element but also on the image receiving element. Therefore, the problem cannot be resolved easily and further research work is required.